1. Field of the Invention
This invention relates to a sealing device for preventing leakage of gas between a piston and a cylinder, the piston being provided with continuous piston rings of plastic material.
2. Description of the Prior Art
Sealing devices of the type referred to are used e.g. in hot gas engines where a lube oil cannot be used because any oil entering into the gas charges of the engine would eventually be accumulated in the engine regenerators and block the gas flow. In double-acting hot gas engines the pistons separate the different gas charges and it is essential that the mean gas pressure is the same in all gas charges of the engine. If not the efficiency of the engine will be lowered.
In prior sealing devices the plastic piston rings have been backed by metal springs to ensure that the plastic rings are held against the surfaces to be sealed. Seals of this type have been described in the U.S. Pat. Nos. 4,132,420 and 4,132,417. The sealing function of said known devices is depending on the magnitude of the forces exerted by the metal springs. However, said forces will vary partly because the springs used in the various pistons are not completely alike, partly because the spring forces will vary dependant on the degree of wear of the plastic piston rings and all rings in all pistons will not be equally worn at any time. Consequently the unavoidable leakage between the pistons and the cyliinder walls will not be of equal magnitude at all pistons of the engine and this will cause different mean pressures in the different working gas charges.